I. Field of the Invention
This invention relates generally to an apparatus for stimulating cardiac tissue, and more particularly to an implantable cardiac rhythm management device and method for automatically adjusting the sensing threshold. The sensing threshold of the cardiac rhythm management device is automatically adjusted on a beat by beat basis following a predetermined period and the adjustment is made dependent upon an amplitude of an intrinsic beat and upon an amplitude and frequency of noise occurring during the period.
II. Discussion of the Prior Art
For the most part, prior art implantable cardiac rhythm management devices, including bradycardia and tachycardia pacemakers and cardiac defibrillators, have sense amplifier circuits for amplifying and filtering electrogram signals picked up by electrodes placed in or on the heart and which are coupled by suitable leads to the implantable cardiac rhythm management device. Typically, the signals emanating from the sense amplifier are applied to one input of a comparator circuit whose other input is connected to a source of reference potential. Only when an electrogram signal from the sense amplifier exceeds the reference potential threshold will it be treated as a cardiac paced or sensed beat. The source reference potential may be referred to as a sensing threshold.
In the case of a programmable cardiac rhythm management device, the prescribing physician can change the threshold potential of the comparator, but in spite of the flexibility which the programmable threshold offers, malsensing of cardiac depolarization will still occur frequently enough to result in patient discomfort. This may be due to the fact that cardiac depolarization events (intrinsic beats) can result in widely different peak amplitudes, depending on patient activity, body position, drugs being used, etc. Lead movement and noise may further impede the detection of cardiac depolarization events. Noise sources may include environmental noise, such as, 50 or 60 Hz power line noise, muscle noise, motion artifacts, baseline wander and T-waves. When the peak amplitudes associated with cardiac depolarization events become too small relative to a programed threshold, or when noise levels in the electrocardiogram approach the sensing threshold, the likelihood of false sensing increases. The peak amplitudes of the electrogram signals associated with cardiac depolarization events is also quite dependent on the type of lead being used and whether the electrodes on the lead abut cardiac tissue or are floating within a cardiac chamber. Thus, it is desirable that an implantable pacemaker or defibrillator be able to automatically adjust the sensing threshold on a beat by beat basis as the patient's activity, body position, drugs being used, etc. changes. It would also be desirable for the rhythm management device to use active or passive fixation bipolar and unipolar leads and/or single pass VDD or DDD leads.
The Jacobson et al. U.S. Pat. No. 5,564,430 describes an automatic sensing threshold control for implantable cardiac rhythm management devices. In the device described by Jacobson et al., the sensing threshold is set according to the amplitude of both sensed signals corresponding to cardiac activity and to sensed noise. The amplitude of sensed noise is measured during a noise interval that follows an absolute refractory period. In many cases there is a need for the sensing threshold to be adjusted prior to the end of both a refractory period and a noise interval, however, the Jacobson device does not provide for such a circumstance. After both independent intervals time out, the Jacobson et al. algorithm adjusts the threshold such that the sensing threshold is substantially below the peak amplitude of sensed cardiac activity, but above the noise level. In accordance with Jacobson et al., the sensing threshold is initially set at a predetermined fraction (one-fourth) of a measured sensed depolarization signal. An absolute refractory period following the occurrence of a sensed signal is provided for, and then, following the absolute refractory period is an established "noise refractory period". If a sensed signal that exceeds the set sensing threshold is detected during the noise refractory period, the noise refractory period is restarted and the sensing threshold is incremented by increasing its level by about 0.2 millivolts such that the resulting sensing threshold will ultimately be set above the noise amplitude. In fluctuating noise environments, such incrementation may not effectively adjust the sensing threshold.
While the method described in the Jacobson et al. patent may prove satisfactory for robust electrocardiogram signals, it is less than satisfactory for low amplitude electrocardiogram signals, such as may be associated with the use of lead pacing systems or low amplitude atrial electrocardiogram signals derived from conventional leads. A further drawback of the algorithm described in the Jacobson et al. '430 patent is the noise detection utilizing retriggerable noise refractory windows. If noise signals are stable in amplitude, the algorithm will detect noise levels properly. However, those skilled in the art appreciate that noise does not behave in that fashion. The amplitude of the noise level keeps changing and often includes impulsive noise excursions. It is, therefore, difficult for a noise refractory window to detect episodes of that type. Also, the delay in adequate threshold adjustment and the 0.2 millivolt buffering between noise and the sensing threshold may not be sufficient in many situations.
A need, therefore, exists for an autosense algorithm for incorporation into a cardiac rhythm management device that will provide automatic adjustment of the sensing threshold, taking into account the amplitude of noise (detected during a predetermined period), and the amplitude of electrocardiogram signals associated with sensed events. The present invention meets that need.